As an example, a prior art PWM inverter having a current control circuit has a block configuration shown in FIG. 2. The DC power is taken via a rectifier 2 from an AC power source 1 and fed to an inverter main circuit 3, which supplies AC power with a PWM waveform having adjusted fundamental frequency and voltage to an induction motor 4 as a load. The control device of inverter main circuit 3 comprises a speed control amplifier 6 which is responsive to a difference signal on a line 6c from a summing junction 6d which compares a speed setting signal (Ns) on a line 6a and a detected signal on a line 6b from a speed detector 5 coupled to motor 4. The amplifier 6 performs a PI operation and provides an amplified signal on a line 6e to a sinusoidal wave generating circuit 7 which generates a sinusoidal signal on a line 7a for adjusting the frequency and voltage according to the speed instruction of amplifier 6. A current control amplifier 9 which amplifies (i.e., performs PI operation) an error signal on a line 9a from a summing junction 9b indicative of a difference between the magnitude of the current instruction signal on line 7a and a detected output current signal on a line 8a from a current detection circuit 8 responsive to the current provided by inverter main circuit 3 to the motor 4 by means of a sensor 8b. A PWM circuit 10 is responsive to an amplified difference signal on a line 9b from the amplifier 9 and provides PWM gating signals on a plurality of lines indicated generally by a line 10a in order to effect an appropriate output waveform for driving the motor by performing ON/OFF control for a corresponding plurality of switch elements in various phases of inverter main circuit 3.
In the conventional PWM inverter having a current control system, when there exists an offset in the current detector and current instruction (sinusoidal wave) that form the current control system, the offset voltage makes the output current of inverter main circuit 3 deviate from the ideal AC waveform as a DC component is overlapped on it, thus causing torque ripple and other problems.
In the conventional scheme, in order to compensate for the offset, as the motor is stopped, the offset amounts in the current instruction and the current detection signal are detected, and compensation signals corresponding to these detected offset amounts are applied during operation.
However, in long-term continuous operation, the offset amount varies as the temperature drifts. In this case, correction becomes impossible. In particular, the temperature drift is large for the current detector, and it is difficult to correct the offset caused by the current detection system. As temperature variation is also caused by seasons, proper correction is impossible. This is a problem.